1. Field of Invention
This invention relates to a new and improved method and compensation circuitry for sensing the value of a direct current (DC) component superimposed on an alternating current (AC) being measured by a watthour meter installation and for deriving and feeding back a feedback signal for nulling out the effect of the DC component on the supply AC being measured.
More particularly, the invention relates to a relatively simple and low cost method of zero crossing detection and electronic compensation and circuitry for achieving compensation of a DC component superimposed on a supply alternating current being measured in a watthour meter installation.
2. Background Problem
It has become a practice of certain consumers of electricity who wish to lower their utility bills to resort to various measures which seriously affect the capability of a watthour meter installation to obtain a true measure of the electricity being consumed. For example, if a diode is placed in series with the load at a consumers dwelling or commercial installation, a direct current component is superimposed on the alternating current flowing through the load. If the particular watthour meter installation in question is designed such that it employs a current transformer for sensing the value of the supply alternating current, the direct current component superimposed on the alternating current can result in a DC magnetizing current flowing in the current transformer which tends to saturate the core of the transformer. If saturation does occur, the current transformer no longer is capable of operating over the linear portion of its magnetization characteristic and is incapable of inducing a signal in the secondary winding of the transformer which is truely proportional to the input AC. Hence, if the current transformer is part of an electronic solid state watthour meter, and its core is saturated in the above manner, the watthour meter is rendered ineffective for accurately measuring the power being consumed by the load at the installation in question.
U.S. patent application Ser. No. 33,078 filed Apr. 25, 1979 in the name of Miran Milkovic, the inventor of the present invention, for "MEANS FOR AUTOMATICALLY COMPENSATING DC MAGNETIZATION IN A TRANSFORMER" and assigned to the General Electric Company, the assignee of the present invention, describes a new and improved compensating means for counteracting direct current saturation of a current transformer in an electronic watthour meter. However, the means described in application Ser. No. 33,078 is relatively complex in that it requires the use of two current transformers and relatively expensive Hall elements.
Copending U.S. patent application Ser. No. 078,058 (GED-1098) filed Sept. 24, 1979, in the name of the present inventor Miran Milkovic and entitled "PEAK DETECTION AND ELECTRONIC COMPENSATION OF D.C. SATURATION MAGNETIZATION IN CURRENT TRANSFORMERS USED IN WATTHOUR METER INSTALLATIONS," also assigned to the General Electric Company, describes and claims a relatively low cost peak detection method and circuit employing only a single current transformer for sensing the value of a DC component superimposed on an alternating current supply being measured by a watthour meter. This system employs a peak detector circuit for deriving a current feedback signal used to null out the effect of the superimposed DC component on the AC being measured.
The present invention describes a new and improved, low cost, method and simplified zero crossing detection technique which also employs a single current transformer for sensing the value of a direct current component superimposed on an alternating current supply being measured by a watthour meter and a highly accurate zero crossing detection circuit that derives a feedback signal representative of the superimposed DC component for nulling out the effect of the DC component on the supply AC being measured.